1. Field of the Invention
The present invention relates to a magneto-optical recording medium for magnetically recording, reproducing and erasing information through irradiation of a laser beam to a magnetic layer therein, and also to a reproducing method for the magneto-optical recording medium.
2. Description of Related Art
Recently, a magneto-optical recording medium with a packing density of approximately 100 times as high as that of a flexible disk has been commercially available. Furthermore, Japanese Patent Application Laid-Open No. 62-184644 (1987) discloses a magneto-optical memory medium basically constituted of an exchange-coupled double-layer film comprising a recording layer made of an antiferromagnetic material and a reading layer made of a ferromagnetic material. In this magneto-optical recording medium, recorded data is very stable against an external bias magnetic field and can be overwritten with ease.
A storage capacity of a 3.5-inch magneto-optical disk is, however, 128 megabyte (MB) or 230 MB, which cannot be said to be sufficient. A much larger capacity is required to deal with dynamic images and the like in multimedia. As a method of increasing the capacity of a magneto-optical disk, a mark length is decreased. The decrease of the mark length has, however, a limitation because there is a limitation in the detection of a laser beam used for reproducing data.
As a method of coping with the limitation in the detection of a reproducing laser beam, a recording method using a signal with a short mark length, which is designated as a magnetically induced super resolution (MSR) method, is proposed. The MSR method utilizes an exchange-coupled force or a magnetostatic-coupled force acting between magnetic layers. FIG. 1 shows the configuration of a general magneto-optical disk device using the MSR method. A magneto-optical disk 30 consists of a magnetic triple-layer 31 formed on a transparent substrate 11, and is rotated by a spindle motor 32. An electromagnet 34 is driven by an electromagnet driving circuit 36 so as to apply a bias magnetic field in a predetermined direction to the magneto-optical disk 30. A direction of the bias magnetic field is switched to be upward or downward depending upon a direction of a current flowing through the electromagnet 34.
A data signal to be written in the magneto-optical disk 30 is generated by a signal generating circuit 38 and supplied to a laser driving circuit 33. In response to the data signal, the laser driving circuit 33 modulates and drives a laser diode 35 in accordance with the data signal. A laser beam generated by the laser diode 35 is collimated by a collimator lens 37, passes through a beam splitter 39, and is focused on the magnetic layer 31 of the magneto-optical disk 30 by an object lens 40. The data signal is thus written in the magneto-optical disk 30 by irradiating the laser beam generated by the laser diode 35 to the magnetic layer 31 on the magneto-optical disk 30 while applying a bias magnetic field in the predetermined direction by the electromagnet 34.
When information (a data signal) recorded in the magneto-optical disk 30 is reproduced, a reproducing laser beam having a smaller intensity than that of a recording laser beam is shed on the magneto-optical disk 30 by driving the laser diode 35 while applying a bias magnetic field in a predetermined direction by the electromagnet 34. Reflected light beam from the magneto-optical disk 30 is collimated by the object lens 40, is reflected by the beam splitter 39, passes through an analyzer 41 and is converged by a lens 42 on an optical sensor 43. The information recorded in the magneto-optical disk 30 is converted into an electric signal by the optical sensor 43 to be reproduced.
FIG. 2 is a schematic sectional view representing the magneto-optical disk 30 used in the MSR method. This conventional magneto-optical disk comprises a dielectric layer 12 and a magnetic triple-layer film including a reproducing layer 13, an intermediate layer 14 and a recording layer 15, and a protective layer 16 formed on a substrate 11 in this order. The intermediate layer 14 controls an exchange-coupled force or a magnetostatic-coupled force between the reproducing layer 13 and the recording layer 15. When the intermediate layer 14 is formed of a magnetic material, the intermediate layer 14 serves as follows: at a temperature below the Curie temperature of the intermediate layer 14, the exchange-coupled force works between the reproducing layer 13 and the recording layer 15, and at a temperature exceeding the Curie temperature, the exchange-coupled force does not work. When the intermediate layer 14 is formed of a non-magnetic material, a magnetostatic-coupled force works between the reproducing layer 13 and the recording layer 15. Table 1 lists general materials, thicknesses and the Curie temperatures of the respective layers, in which those adopted when the intermediate layer 14 is formed of a non-magnetic material are described in brackets.
TABLE 1 ______________________________________ Thickness Curie Temp. Material (nm) (.degree. C.) ______________________________________ Protection SiN 100 -- Layer Recording Tb.sub.21 Fe.sub.64 Co.sub.15 30 250 Layer Intermediate Tb.sub.22 Fe.sub.78 (SiO.sub.2) 15 (2) 130 (-) Layer Reproducing Gd.sub.24 Fe.sub.56 Co.sub.20 30 .apprxeq.300 Layer Dielectric SiN 100 -- Layer ______________________________________
Now, the principle of the reproducing operation in the MSR method of an exchange-coupled type through the control of the exchange-coupled force by the intermediate layer 14 will be described. FIG. 3 is a schematic diagram showing a state of magnetization in each of the magnetic layers, i.e., the reproducing layer 13, the intermediate layer 14 and the recording layer 15. The magneto-optical disk is rotated in a direction shown with a white arrow, to which is irradiated a reproducing laser beam from the reproducing layer 13 side during the reproducing operation. With the reproducing laser beam, a front portion 17 seen from the rotating direction of the magneto-optical disk in an area hit by the reproducing laser beam is heated to not lower than the Curie temperature of the intermediate layer 14. When a reproducing magnetic field in a direction shown with an arrow 19 is applied from external by the electromagnet 34, the magnetization of the heated portion 17 in the reproducing layer 13 is aligned in one direction, i.e., the direction shown with the arrow 19. Since the front portion 17 of the reproducing layer 13 in which the magnetization is aligned in one direction works as a mask, marks present in this portion are not detected. Therefore, a rear portion 18 which has a lower temperature in the area hit by the reproducing laser beam serves as an aperture, so that marks recorded in the rear portion 18 in the reproducing layer 13 are detected and reproduced. In the aperture, no interface magnetic domain wall is produced between the magnetic layers, and hence the magnetization is kept stable. Since only marks recorded in the aperture (i.e., the rear portion 18) in the area hit by the reproducing laser beam can be detected in this manner, it is possible to read a smaller mark than in the conventional device.
Furthermore, Japanese Patent Application Laid-Open No. 5-89536 (1993) discloses a magneto-optical recording medium having, between a substrate and a recording layer, a reproducing layer made of a material whose phase is changed to be antiferromagnetic with an increase in temperature. This magneto-optical recording medium attains a stable shape of a magnetic domain for the reproducing operation, and is capable of stably MSR reproducing information in a wide range of the operation temperature.
In overwriting recorded data in this magneto-optical recording medium, it is necessary to first erase the existing data by applying an erasing magnetic field by an electromagnet and then to record new data through the conversion of the magnetic field to a recording magnetic field. Therefore, the magneto-optical recording medium disadvantageously requires a longer time to overwrite data. In order to overcome this disadvantage and increase a data transfer speed, various types of direct overwritable magneto-optical recording media utilizing the intensity modulation of a laser beam are proposed.
FIG. 4 is a sectional view showing the structure of a direct overwritable magneto-optical recording medium. This magneto-optical recording medium comprises a (bottom) dielectric layer 22, a memory layer 23 for signal reproduction, an intermediate layer 24 used for controlling an exchange-coupled force between the memory layer 23 and a recording layer 25, the recording layer 25 used for recording or erasing data, a switching layer 26 used for controlling an exchange-coupled force between the recording layer 25 and an initializing layer 27, the initializing layer 27 used for retaining a direction of magnetization in an erasing operation, and a (top) protective layer 28 that are layered on a substrate 21 in this order. Table 2 below lists typical materials, thicknesses, Curie temperatures and film forming conditions for the respective layers in this magneto-optical recording medium. The total thickness of the magnetic film (i.e., the memory layer 23, the intermediate layer 24, the recording layer 25, the switching layer 26 and the initializing layer 27) of the exemplified recording medium is 150 nm.
TABLE 2 ______________________________________ Forming Condition Curie Ar Gas Thickness Temp. Pres- Power Material (nm) (.degree. C.) sure(Pa) (kW) ______________________________________ Protective SiN 100 -- 0.5 1.0 layer Initializing Tb.sub.30 Co.sub.70 40 &gt;300 Layer Switching Tb.sub.20 Fe.sub.80 15 130 Layer Recording Tb.sub.5 Dy.sub.25 Fe.sub.35 Co.sub.35 40 250 Layer 0.5 1.0 Intermediate Gd.sub.30 Fe.sub.50 Co.sub.20 15 .apprxeq.300 Layer Memory Tb.sub.20 Fe.sub.71 Co.sub.9 40 200 Layer Dielectric SiN 100 -- 0.5 1.0 Layer ______________________________________
In recording data, the switching layer 26 hit by the laser beam is heated to not lower than the Curie temperature thereof, and hence, no exchange-coupled force works between the recording layer 25 and the initializing layer 27. Under this condition, a bias magnetic field in a direction reverse to that of an erasing magnetic field is applied, whereby data is recorded in the recording layer 25. When the temperature is decreased to not higher than the Curie temperature of the intermediate layer 24, an exchange-coupled force works between the memory layer 23 and the recording layer 25 thereby to copy the data to the memory layer 23. When the temperature is further decreased and becomes not higher than the Curie temperature of the switching layer 26, an exchange-coupled force works between the recording layer 25 and the initializing layer 27 to align the magnetization in the recording layer 25 with a direction for the erasing operation (i.e., the magnetization direction in the initializing layer 27). Since the recording layer 25 returns to the erasing state after the data recorded therein is transferred to the memory layer 23 via the intermediate layer 24 in this manner, data can be directly overwritten without a need for separate execution of an erasing operation. Therefore, the recording layer 25 is indispensable for the direct overwriting operation.
Thus, in the magneto-optical recording medium having the film structure as shown in Table 2, the intermediate layer 24 and the switching layer 26 function to control an exchange-coupled force between the respective adjacent layers. In this case, it is necessary that the exchange-coupled force to be controlled by the switching layer 26 is larger than the exchange-coupled force to be controlled by the intermediate layer 24 around room temperature. The reason is as follows: it is necessary to copy data in the recording layer 25 to the memory layer 23 at a temperature not lower than the Curie temperature of the switching layer 26 and to forcibly align the magnetization direction in the recording layer 25 with the magnetization direction in the initializing layer 27 when the temperature of the switching layer 26 is decreased to not higher than the Curie temperature so as to retain the erasing state. Therefore, the exchange-coupled force to be controlled by the intermediate layer 24 is required to be sufficiently strong to ensure the copying at a temperature not lower than the Curie temperature of the switching layer 26 and to be sufficiently smaller (for example, half or smaller) than the exchange-coupled force to be controlled by the switching layer 26 at a temperature not higher than the Curie temperature of the switching layer 26.
Now, various types of conventional direct overwriting magneto-optical recording media will be enumerated.
A magneto-optical recording medium disclosed in Japanese Patent Application Laid-Open No. 3-93056 (1991) comprises a magneto-optical recording layer and a perpendicularly magnetized film used for magnetic recording both formed on a substrate. The magneto-optical recording layer includes a copying layer in which a recording signal in the perpendicularly magnetized film is copied and a reproducing layer magnetically bound to the copying layer and used for converting the recording signal into an optical signal by the magneto-optical effect. Data is written in the perpendicularly magnetized film by using a magnetic head, and a signal recorded in the perpendicularly magnetized film is copied, through irradiation of a laser beam, to the copying layer that is bound to the perpendicularly magnetized film by an exchange-coupled or magnetostatic-coupled force. Furthermore, in the reproducing layer magnetically bound to the copying layer, a magnetic domain pattern corresponding to the copying layer is formed. In reproducing data, a magnetization signal recorded in the reproducing layer is converted into an optical signal and read out through the irradiation of a laser beam. Thus, data in this magneto-optical recording medium can be directly overwritten by a magnetic head.
A magneto-optical recording medium disclosed in Japanese Patent Application Laid-Open No. 3-156751 (1991) has a four-layer structure including a recording layer, a recording auxiliary layer, a control layer and an orientation layer. The adjacent magnetic layers are bound to each other through an exchange-coupled force, and the recording layer (a first magnetic layer) has perpendicular magnetic anisotropy. A laser beam which is modulated to have binary intensities is shone to the magneto-optical recording medium. The orientation layer (a fourth magnetic layer) has an exchange-coupled force antagonistic to a magnetostatic-coupled force, and also has a function similar to that of an external magnetic field. Therefore, there is no need to separately apply an external magnetic field.
A magneto-optical recording medium disclosed in Japanese Patent Application Laid-Open No. 3-156752 (1991) has a similar four-layer structure, in which a control layer (a third magnetic layer) has an extremely small thickness and a low Curie temperature. Therefore, the third magnetic layer is greatly influenced by an exchange-coupled force, and an exchange-coupled force from a fourth magnetic layer acts on a second magnetic layer via the third magnetic layer. As a result, a direct overwriting operation can be smoothly conducted with an extremely small initialization magnetic field.
Moreover, Japanese Patent Application Laid-Open No. 3-230339 (1991) discloses a magneto-optical recording medium with an intermediate layer for controlling an exchange-coupled force between a first magnetic layer and a second magnetic layer. In forming the intermediate layer, impurities are mixed therein so as to increase an apparent Kerr rotation angle, thereby decreasing the exchange-coupled force.
A magneto-optical recording medium disclosed in Japanese Patent Application Laid-Open No. 4-192135 (1992) is provided with a layer of rare earth rich sublattice magnetization between a first magnetic layer and a second magnetic layer. Owing to this layer, an exchange-coupled force between the magnetic layers is decreased, and the magnetic layers can be stably formed.
A magneto-optical recording medium disclosed in Japanese Patent Application Laid-Open No. 5-6588 (1993) has a recording layer comprising a memory layer made of a ferromagnetic film having perpendicular magnetic anisotropy and an auxiliary layer formed on the memory layer and made of an antiferromagnetic film which is in the antiferromagnetic phase at room temperature and is changed to be ferromagnetic through magnetic phase transition around the Curie temperature of the memory layer higher than room temperature. The auxiliary layer made of the antiferromagnetic film is used as a layer for assisting a data recording operation.
In the MSR method utilizing an exchange-coupled force, a magnetic layer made of a rare earth-transition metal amorphous alloy film is generally used as the intermediate layer 14. The rare earth-transition metal amorphous alloy film is a ferrimagnetic material and is a kind of ferromagnetic material. Therefore, an interface magnetic domain wall appears between the reproducing layer 13 and the intermediate layer 14 or between the intermediate layer 14 and the recording layer 15. Such a multi-layer medium as above has a hysteresis loop symmetrical with respect to an origin as shown in FIG. 5A. However, the hysteresis loop becomes asymmetric due to the exchange-coupled force, that is, a so-called hysteresis loop shift as shown in FIG. 5B occurs depending upon whether or not the interface magnetic domain wall is formed by the exchange-coupled force between the reproducing layer 13 and the recording layer 15. FIG. 6 shows temperature characteristics of the hysteresis loop shift observed when the intermediate layer 14 is formed of a rare earth-transition metal amorphous alloy. As shown in FIG. 6, an actual value attains a larger magnetic field than a target value in the temperature range from room temperature to a given temperature, and hence, a large reproducing magnetic field is required to reverse the magnetization in the reproducing layer 13 for the formation of a mask. Therefore, it is impossible to achieve an MSR with a reproducing laser beam having a small intensity.
In the MSR method utilizing a magnetostatic-coupled force, the force is too weak to sufficiently copy data from the recording layer 15 to the reproducing layer 13. This method also has a problem that a noise level of a recording signal is increased because of instability, in particular, around a mark.
Also in the direct overwritable magneto-optical recording medium shown in FIG. 4, a material for the intermediate layer 24 is a rare earth-transition metal amorphous alloy. Therefore, the exchange-coupled force between the memory layer 23 and the recording layer 25 may be stronger than required. The exchange-coupled force .sigma..sub.W (which is also denoted as an interface magnetic domain wall energy) is represented by the following equation: EQU .sigma..sub.W =H.sub.S .times.(2M.sub.S d)
wherein H.sub.S is a hysteresis loop shift amount; M.sub.S is a saturation magnetization; and d is a thickness of a film. The exchange-coupled force .sigma..sub.W between the memory layer and the recording layer in the film structure listed in Table 2 is 1 to 2 erg/cm.sup.2. When the thickness d is increased, the loop shift amount H.sub.s is decreased, but a recording sensitivity is degraded resulting from the increase of the thickness d.
In addition, in the conventional magneto-optical recording medium, when the reproducing power is large (for example, 1.5 mW or more), data in the memory layer 23 sometimes starts to be erased in accordance with the magnetization direction (erasing direction) in the recording layer 25. Such a phenomenon as above takes place when the exchange-coupled force between the memory layer 23 and the recording layer 25 is strong and the loop shift amount of the hysteresis loop is large. Specifically, when the loop shift amount is large, the temperature of the magneto-optical recording medium is increased through the irradiation of the reproducing laser beam thereby to decrease a coercive force. As a result, the exchange-coupled force is increased, and the magnetization is reversed in the absence of the magnetic field to be changed in the magnetization direction of the erasing time. This phenomenon decreases a C/N ratio, thereby degrading a reproducing stability.